1. Field of the Invention
The present invention relates to a flexographic printing plate mounting sheet that doubles as a cushioning element used between a flexographic printing plate and an impression cylinder in flexographic printing.
2. Description of the Related Art
In flexographic printing, a flexographic printing plate is mounted to a plate cylinder of an impression cylinder to produce a printed copy. Of various conventional methods that have been proposed to mount a flexographic printing plate to an impression cylinder, double-sided adhesive sheets for mounting a press plate have been popular lately because of their ease of use and ability to achieve accurate printing. Such double-sided adhesive sheets for mounting a press plate are obtained by forming an adhesive layer on each side of a substrate made from an elastic, foamed body that has resilience against compression, and double as a cushioning element between the flexographic printing plate and the impression cylinder.
FIG. 2 is a partial sectional view showing an example of prior art. In FIG. 2, a flexographic printing plate 19 is mounted to an impression cylinder 18 with the use of a double-sided adhesive sheet 11 for mounting a press plate. The double-sided adhesive sheet 11 for mounting a press plate is composed of a foamed substrate 12, which has adhesive layers 13 and 14 formed on its respective sides. The flexographic printing plate 19 is stuck to the adhesive layer 13 on one side and the impression cylinder 18 is stuck to the adhesive layer 14 on the other side. In this way, the flexographic printing plate 19 can readily be attached and mounted to the impression cylinder 18.
However, there are some inconveniences to the conventional double-sided adhesive sheet 11 for mounting a press plate in which the foamed substrate 12 has the adhesive layers 13 and 14 formed on its respective sides. When stripping the double-sided adhesive sheet 11 from the impression cylinder 18, the substrate 12 and the adhesive layer 14 could be delaminated leaving, on the impression cylinder 18, the adhesive and/or small pieces torn off of the foamed substrate 12 and adding a labor of cleaning the residues. Furthermore, the double-sided adhesive sheet 11 cannot be used repeatedly with the foamed substrate 12 torn upon peeling off of the adhesive sheet and with the adhesive layer 14 made uneven in thickness by the adhesive left on the impression cylinder 18.
In an attempt to counter these problems, a flexographic printing plate mounting sheet (see JP 10-501192 A, for example) and a double-sided adhesive sheet for mounting a press plate with an intervening plastic film serving as a reinforcing sheet (see BP 1533431 B), for example) have been proposed.
FIG. 3 is a frontal explanatory diagram showing a partially enlarged view of a double-sided adhesive sheet for mounting a press plate with an intervening plastic film serving as a reinforcing sheet. In FIG. 3, a double-sided adhesive sheet 11 for mounting a press plate is composed of a foamed substrate 12, which has an adhesive layer 14 on one side and an adhesive layer 17 and a plastic film 15 on the other side. The remaining side of the plastic film 15 has an adhesive layer 13, which is protected by a release liner (release paper) 16. With the side on which the plastic film 15 is formed fixed to an impression cylinder 18, the double-sided adhesive sheet 11 for mounting a press plate can be peeled off without tearing the foamed substrate 12 or leaving an adhesive residue owing to the presence of the plastic film 15.
In flexographic printing of which higher printing accuracy is demanded, a flexographic printing plate has to meet strict standards for thickness accuracy and positioning accuracy. Accordingly, a flexographic printing plate mounting sheet too is required to have uniform thickness accuracy as well as appropriate cushioning properties and, upon installation, the ease of positioning relative to an impression cylinder without fail.
In light of this, none of conventional flexographic printing plate mounting sheets, which use as a foamed substrate a slice of polyethylene or other thermoplastic foam with a width of 30 to 1500 mm and a thickness of 0.3 to 0.5 mm or a thin urethane sheet coated with an adhesive on each side, have uniform thickness accuracy. The thickness fluctuation lowers the printing accuracy and gives the printed copy poor quality.
Furthermore, conventional flexographic printing plate mounting sheets are usually opaque and obscure positioning marks on an impression cylinder, which makes correct positioning on the marked impression cylinder difficult. With a flexographic printing plate mounting sheet positioned incorrectly on an impression cylinder, a flexographic printing plate is mounted misaligned to the impression cylinder and the printing accuracy is lowered.
Moreover, conventional flexographic printing plate mounting sheets lose appropriate cushioning properties through repeated use since their foamed substrates do not have proper distortion characteristics, thereby cutting short their life spans.